Protein to DNA synthesis using antiphage reverse transcriptase

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Protein-Templated DNA Synthesis: Bacteria totally Just Broke a Fundamental Rule of Molecular Biology

Reverse syntthesis of DNA strand from protein' amino acid sequences via action of Drt3b Credits: ai@FST

Title: Protein-templated synthesis of dinucleotide repeat DNA by an antiphage reverse transcriptase Authors: Pujuan Deng et al. (Stanford University / Alex Gao lab) Journal: Science (First Release, April 16, 2026) DOI: 10.1126/science.aed1656

The Big Deal

For decades, the central dogma of molecular biology has held that nucleic acids (DNA and RNA) are synthesized using other nucleic acids as templates. DNA polymerases copy DNA from DNA; RNA polymerases copy RNA from DNA; reverse transcriptases copy DNA from RNA. Protein → nucleic acid information transfer? Strictly off-limits in the classic formulation.

This new paper shows a bacterial defense enzyme that literally uses its own protein structure as a template to synthesize a specific DNA sequence. It's not full reverse translation of amino acid sequence into a gene, but it's a stunning workaround that produces precise, alternating dinucleotide DNA without any nucleic acid template guiding one of the strands.

What the DRT3 System Does

Bacteria use Defense-associated Reverse Transcriptases (DRTs) as anti-phage weapons. The DRT3 variant (found in E. coli and others) assembles into a beautiful D3-symmetric hexamer: 6 copies each of Drt3a, Drt3b, and a non-coding RNA (ncRNA).

  • Drt3a behaves more conventionally: It uses a short ACACAC repeat in the ncRNA as a template to make a poly(GT) DNA strand.
  • Drt3b is the star: It synthesizes the complementary poly(AC) strand with no nucleic acid template at all. Instead, specific amino acid residues in its active site act like a "protein template," enforcing strict alternation of A and C through precise base interactions and a protein-primed mechanism.

The result? Double-stranded DNA with repeating GT/AC dinucleotides. This repetitive DNA is produced even without phage infection and likely acts as a decoy or disrupts viral replication.

Cryo-EM structures (up to 2.6 Å resolution) of the elongating and resting states beautifully capture this machinery in action, with PDB codes 9Z6Y and 9Z6Z.

Why This Matters

  • Breaks the template dogma (with caveats): It shows proteins can guide sequence-specific polymerization in a highly controlled way. The enzyme doesn't "read" its own amino acid sequence like a code, but uses its 3D structure and key residues to stamp out a repetitive sequence.
  • Bacterial immunity innovation: Adds to the growing arsenal of weird anti-phage systems (think CRISPR, but this one is RNA-protein-DNA acrobatics).
  • Broader implications: Expands what we think polymerases can do. Could inspire new synthetic biology tools for making repetitive DNA or novel enzymes. Also fuels fun (and sometimes overblown) discussions about information flow in biology.

This is elegant, surprising biochemistry. Evolution found a way for a protein to template DNA synthesis for defense, using structural mimicry instead of base-pairing to a nucleic acid strand. It's not rewriting the central dogma entirely, but it definitely adds a fascinating asterisk.

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